Devices for detecting the presence of water or ice on structures, for example aircraft structures, are known. Example devices include those that detect ice formation using optical means (i.e. detecting a change in opacity or refractive index around a sensor). Others include those that monitor changes in a resonant frequency of a structure (i.e. the accumulation of ice on a structure alters its resonant frequency).
Preferably, ice detection methods only detect the impingement of water that is likely to ‘stick’ to the surface of a structure. Water crystals, for example, usually bounce off a structure, and thus are less likely to accrete on a surface and thus are unlikely to cause problems.
Some devices used to detect the presence of water or ice on a structure include Pitot-type detectors, which ingest all forms of water, including crystals (which usually just bounce off the structure). They cannot distinguish between ‘sticky’ and ‘non-sticky’ water. Pitot-type detectors can also become blocked (and need regular cleaning).
In GB 2483530, we described a scheme in which the ice detection system was driven to detect an icing condition based on the power required to heat first and second heaters to different temperatures in a temperature range. However, it has been found that the described schemes sometimes do not detect icing conditions due to heat spread through the skin and due to insufficient temperature measurement accuracies.
There are also developments in the FAA regulations on Supercooled Large Droplets (forming Appendix O of Part 25), which will require systems to detect their presence. Many of the above-mentioned methods are not appropriate for detecting such droplets.
We have therefore appreciated the disadvantages of known devices for detecting the presence of water or ice on the surface of a structure (including SLDs), and the need for an improved method and device.